There are sports that require physical pounding (football), psychological toughness (tennis), and mind-blowing hand-eye coordination (baseball), but the sports that demand the very highest physiological output, by my estimation, are cycling and cross-country skiing. Both have some of the highest caloric requirements – 6,500 a day for your average Tour de France rider and more than 1,000 an hour for a competitive cross-country skier. They also boast the top two VO2 maxes (a measurement of how much oxygen a body uses during exercise) ever recorded, and share bragging rights for Fittest Person on the Planet. The current record holder for VO2 max is pro cyclist Oskar Svendsen, and I'm not surprised his sport tops the list: I find fever-pitch bike racing to be the hardest imaginable sport, in which you are constantly pressed to push the furthest limits of your body.